afs: Give an afs_server object a ref on the afs_cell object it points to

afs: Give an afs_server object a ref on the afs_cell object it points to

Give an afs_server object a ref on the afs_cell object it points to so that
the cell doesn't get deleted before the server record.

Whilst this is circular (cell -> vol -> server_list -> server -> cell), the
ref only pins the memory, not the lifetime as that's controlled by the
activity counter.  When the volume's activity counter reaches 0, it
detaches from the cell and discards its server list; when a cell's activity
counter reaches 0, it discards its root volume.  At that point, the
circularity is cut.

Fixes: d2ddc776a458 ("afs: Overhaul volume and server record caching and fileserver rotation")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250218192250.296870-6-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>